This invention generally relates to lamp driver circuits for controlling power supplied to a lamp from a power supply subject to voltage variations and, more particularly, to a lamp driver circuit which senses the current through, and the voltage applied to the lamp to control the power supplied to the lamp, and which pulse-width modulates the power supplied to the lamp when the voltage of the power supply rises above a nominal level.
Current sensing circuits are known to the power supply art. Such circuits generally limit the maximum current which the power supply will deliver under potentially destructive conditions to avoid failure of the power supply. A typical example is to limit the power supply output current when a short circuit is imposed across the output of the power supply. Similarly, various kinds of over-voltage protection circuits exist.
However, in vehicular applications, the battery voltage from which many lamps are operated can vary over significant ranges for a number of reasons. One such reason is that the alternator output current available at low idle speeds is limited. Another reason is that vehicular power systems are typically temperature compensated to regulate the battery voltage at different levels depending upon the ambient temperature. It is also well known that voltage regulator failure can result in an abnormally high voltage level in some vehicular systems. In some vehicular applications, especially in heavier trucks, there is a need to operate from a 24 volt DC system, instead of from the more prevalent 12 volt DC system.
It is also well known that incandescent lamps commonly used in vehicles have a relatively narrow operating voltage range. As a example, a lamp rated at 12 volts will experience burn-out of the filament, or unacceptably short operating life, if such a lamp is operated in a 24 volt system. Conversely, a lamp rated at 24 volts will typically produce an unacceptably low level of illumination if operated in a 12 volt system.